Mini-CAT 1 Final

Rt 2 Wk 1 PICO Expanded into Mini-CAT

Clinical Question:

Arterial blood gas (ABG) measurement has traditionally been the go-to method for assessing a patient’s acid-base status. This method, however, is very uncomfortable for the patient and can be difficult to obtain due to poor pulses or any patient movement, not to mention the potential complications associated with an ABG. Venous blood gas (VBG) measurement via a peripheral, central, or mixed venous sample can provide an alternative method to monitor this that is faster, easier, and certainly more comfortable for the patient, particularly in those who may require repeat measurements. Thus, it is important to assess whether VBG is a reliable measurement of acid-base status in the way that ABG is.

PICO Question:

How accurate is VBG measurement compared to ABG measurement in adult patients?

Search Strategy:

P	I	C	O
Adult patients	Venous blood gas	Arterial blood gas	Accuracy
Adults	VBG	ABG	Detecting acid-base disturbances
	Peripheral VBG		Sensitivity
	Central VBG
	Mixed venous sample
	Peripheral venous gas (PVG)

Google Scholar

• Venous blood gas versus arterial blood gas for detecting acid-base disturbances

• • 16,700 results
• Exclude patents and citations, show results from 2010-2020

• • 8,250 results 

Pubmed

• Accuracy of venous blood gas

• • 193 results
• Filters: Free full text, publication within the last 10 years, human participants
  • 27 results

• Venous blood gas compared to arterial blood gas

• • 1,808 results
• Filters: Free full text, publication within the last 10 years, human participants

Trip Database

• P – Adults
• I – Venous blood gas
• C – Arterial blood gas
• O – Accuracy

• • 661 results

• P – Adults
• I – Venous blood gas
• C – Arterial blood gas
• O – Sensitivity

• • 1,049 results

Cochrane Database

• Venous blood gas accuracy

• • 1 Cochrane protocol, 16 trials

I was able to select my final articles based on these results by determining which articles were relevant to my question based on the abstract. When I selected those, I was further able to narrow down my article choices based on the content of the rest of the articles and select which ones were most relevant to my question and offered the highest and most reliable level of evidence. I gave preference, when possible, to articles published in the United States and within the last 10 years.

Articles Chosen for Inclusion:

1. Correlation of Venous Blood Gas and Pulse Oximetry with Arterial Blood Gas in the Undifferentiated Critically Ill Patient

Zeserson, E., Goodgame, B., Hess, D., Schultz, K., Hoon, C., Lamb, K., Maheshwari, V., Johnson, S., Papas, M., Reed, J., & Breyer, M. (2018). Correlation of Venous Blood Gas and Pulse Oximetry with Arterial Blood Gas in the Undifferentiated Critically Ill Patient. J Intensive Care Med; 33(3), 176-181.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5885755/

Rationale

Blood gas analysis is often used to assess acid–base, ventilation, and oxygenation status in critically ill patients. Although arterial blood gas (ABG) analysis remains the gold standard, venous blood gas (VBG) analysis has been shown to correlate with ABG analysis and has been proposed as a safer less invasive alternative to ABG analysis.

Objective

The purpose of this study was to evaluate the correlation of VBG analysis plus pulse oximetry (SpO₂) with ABG analysis.

Methods

We performed a prospective cohort study of patients in the emergency department (ED) and intensive care unit (ICU) at a single academic tertiary referral center. Patients were eligible for enrollment if the treating physician ordered an ABG. Statistical analysis of VBG, SpO₂, and ABG data was done using paired t test, Pearson χ², and Pearson correlation.

Main Results

There were 156 patients enrolled, and 129 patients completed the study. Of the patients completing the study, 53 (41.1%) were in the ED, 41 (31.8%) were in the medical ICU, and 35 (27.1%) were in the surgical ICU. The mean difference for pH between VBG and ABG was 0.03 (95% confidence interval: 0.03–0.04) with a Pearson correlation of 0.94. The mean difference for pCO₂ between VBG and ABG was 4.8 mm Hg (95% confidence interval: 3.7–6.0 mm Hg) with a Pearson correlation of 0.93. The SpO₂ correlated well with PaO₂ (the partial pressure of oxygen in arterial blood) as predicted by the standard oxygen–hemoglobin dissociation curve.

Conclusion

In this population of undifferentiated critically ill patients, pH and pCO₂ on VBG analysis correlated with pH and pCO₂ on ABG analysis. The SpO₂ correlated well with pO₂ on ABG analysis. The combination of VBG analysis plus SpO₂ provided accurate information on acid–base, ventilation, and oxygenation status for undifferentiated critically ill patients in the ED and ICU.

2. Review Article: Can Venous Blood Gas Analysis Replace Arterial in Emergency Medical Care?

Kelly, A. M. (2010). Review Article: Can Venous Blood Gas Analysis Replace Arterial in Emergency Medical Care? Emerg Med Australas; 22(6), 493-498.

https://www.ncbi.nlm.nih.gov/pubmed/?term=Review+article%3A+Can+venous+blood+gas+analysis+replace+arterial+in+emergency+medical+care

The objectives of the present review are to describe the agreement between variables on arterial and venous blood gas analysis (in particular pH, pCO(2) , bicarbonate and base excess) and to identify unanswered questions. MEDLINE search of papers published from 1966 to January 2010 for studies comparing arterial and peripheral venous blood gas values for any of pH, pCO(2) , bicarbonate and base excess in adult patients with any condition in an emergency department setting. The outcome of interest was mean difference weighted for study sample size with 95% limits of agreement. The weighted mean arterio-venous difference in pH was 0.035 pH units (n= 1252), with narrow limits of agreement. The weighted mean arterio-venous difference for pCO(2) was 5.7 mmHg (n= 760), but with 95% limits of agreement up to the order of ±20 mmHg. For bicarbonate, the weighted mean difference between arterial and venous values was -1.41 mmol/L (n= 905), with 95% limits of agreement of the order of ±5 mmol/L. Regarding base excess, the mean arterio-venous difference is 0.089 mmol/L (n= 103). There is insufficient data to determine if these relationships persist in shocked patients or those with mixed acid-base disorders. For patients who are not in shock, venous pH, bicarbonate and base excess have sufficient agreement to be clinically interchangeable for arterial values. Agreement between arterial and venous pCO(2) is too poor and unpredictable to be clinically useful as a one-off test but venous pCO(2) might be useful to screen for arterial hypercarbia or to monitor trends in pCO(2) for selected patients.

3. Peripheral Venous and Arterial Blood Gas Analysis in Adults: Are They Comparable? A Systematic Review and Meta-Analysis.

Byrne, A. L., Bennet, M., Chatterji, R., Symons, R., Pace, N. L, & Thomas, P. S. (2014). Peripheral venous and arterial blood gas analysis in adults: Are they comparable? A systematic review and meta-analysis. Respirology; 19(2), 168-175.

https://www.ncbi.nlm.nih.gov/pubmed?term=24383789

Peripheral venous blood gas (PVBG) analysis is increasingly being used as a substitute for arterial blood sampling; however, comparability has not been clearly established. To determine if the pH, PCO2 and PO2 obtained from PVBG analysis is comparable with arterial blood gas (ABG) analysis. A search was conducted of electronic databases as well as hand-searching of journals and reference lists through December 2012 to identify studies comparing PVBG with ABG analysis in adult subjects. A systematic review was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses statement. A meta-analysis using a random effects model was used to calculate the average difference (bias) and the limits of agreement for the venous and arterial pH, PCO2 and PO2 . A total of 18 studies comprising 1768 subjects were included in the meta-analysis. There was considerable heterogeneity between studies with I(2) approaching 100%. There was little difference between the pH obtained from the PVBG and the ABG, with the arterial pH typically 0.03 higher than the venous pH (95% confidence interval 0.029-0.038). The venous and arterial PCO2 were not comparable because the 95% prediction interval of the bias for venous PCO2 was unacceptably wide, extending from -10.7 mm Hg to +2.4 mm Hg. The PO2 values compared poorly, the arterial PO2 typically 36.9 mm Hg greater than the venous with significant variability (95% confidence interval from 27.2 to 46.6 mm Hg). PVBG analysis compares well with ABG analysis for pH estimations in adults

4. Predicting Arterial Blood Gas and Lactate From Central Venous Blood Analysis in Critically Ill Patients: A Multicentre, Prospective, Diagnostic Accuracy Study

Boulain, T., Garot, D., Vignon, P, Lascarrou, J. B., Benzekri-Lefevre, D., Dequin, P. F., Clinical Research in Inensive Care and Sepsis (CRICS) Group. (2016). predicting arterial blood gas and lactate from central venous blood analysis in critically ill patients: a multicentre, prospective, diagnostic accuracy study. British Journal of Anaesthesia; 117 (3), 341 – 349.

https://www.ncbi.nlm.nih.gov/pubmed/27543529

BACKGROUND:

The estimation of arterial blood gas and lactate from central venous blood analysis and pulse oximetry [Formula: see text] readings has not yet been extensively validated.

METHODS:

In this multicentre, prospective study performed in 590 patients with acute circulatory failure, we measured blood gases and lactate in simultaneous central venous and arterial blood samples at 6 h intervals during the first 24 h after insertion of central venous and arterial catheters. The study population was randomly divided in a 2:1 ratio into model derivation and validation sets. We derived predictive models of arterial pH, carbon dioxide partial pressure, oxygen saturation, and lactate, using clinical characteristics, [Formula: see text], and central venous blood gas values as predictors, and then tested their performance in the validation set.

RESULTS:

In the validation set, the agreement intervals between predicted and actual values were -0.078/+0.084 units for arterial pH, -1.32/+1.36 kPa for arterial carbon dioxide partial pressure, -5.15/+4.47% for arterial oxygen saturation, and -1.07/+1.05 mmol litre(-1) for arterial lactate (i.e. around two times our predefined clinically tolerable intervals for all variables). This led to ∼5% (or less) of extreme-to-extreme misclassifications, thus giving our predictive models only marginal agreement. Thresholds of predicted variables (as determined from the derivation set) showed high predictive values (consistently >94%), to exclude abnormal arterial values in the validation set.

CONCLUSIONS:

Using clinical characteristics, [Formula: see text], and central venous blood analysis, we predicted arterial blood gas and lactate values with marginal accuracy in patients with circulatory failure. Further studies are required to establish whether the developed models can be used with acceptable safety.

Summary of the Evidence:

Author (Date)	Level of Evidence	Sample/Setting (# of subjects/ studies, cohort definition etc. )	Outcome(s) studied	Key Findings	Limitations and Biases
Zeserson, E., Goodgame, B., Hess, D., Schultz, K., Hoon, C., Lamb, K., Maheshwari, V., Johnson, S., Papas, M., Reed, J., & Breyer, M.   (2018)	Prospective cohort study	129 adult patients from Christiana Hospital in Newark, Delaware who had an ABG ordered à 53 in ED à 41 in MICU à 35 in SICU	·       The correlation between VBG analysis alone and combined with pulse oximetry with ABG analysis ·       The researchers also compared central and peripheral VBG analyses, different diagnosis groups, including shock vs non-shock, CHF, COPD/asthma, pneumonia, non-pulmonary sepsis, and neurologic diagnoses, and the 3 different departments sampled from	·       The mean difference for pH between VBG and ABG was 0.03 (95% CI: 0.03–0.04) with a Pearson correlation (r) of 0.94 ·       The mean difference for pCO2 between VBG and ABG was 4.8 mm Hg (95% CI: 3.7–6.0 mm Hg), r = 0.93. ·       The SpO2 correlated well with PaO2 as predicted by the standard oxygen–hemoglobin dissociation curve ·       The researchers found strong positive correlations between venous and arterial pH and pCO2 in general, with the former being favored in central VBG and the latter being favored in peripheral VBG ·       They also found a strong positive correlation between SpO2 and pulse oximetry reading, although 7 patients’ readings did not correlate with the SpO2 produced by the ABG ·       The researchers found that the correlations between ABG and VBG were least strong in patients with neurological diagnoses ·       Overall, VBG alone may be deficient in comparison with ABG, but is adequately comparable with the supplementation of pulse oximetry.	·       Small sample size (n = 129) à less power in the result; amplified because not all 129 participants received the same type of VBG, though all received an ABG ·       Potential sampling bias because all patients were enrolled during daytime hours on weekdays, excluding patients that presented overnight or on weekends; offset by the diverse backgrounds of the participants
Kelly, A. M.   (2010)	Systematic review	·       MEDLINE search of papers published from 1966 to January 2010 for studies comparing arterial and peripheral venous blood gas values for any of pH, pCO(2) , bicarbonate and base excess in adult patients with any condition in an emergency department setting ·       A repeated search was conducted in May 2010 to identify any further papers published during data collection and analysis. The terms ‘arterial’, ‘venous’, ‘blood gas’ and ‘agreement’ were used and the search was limited to ‘adults’. In addition, similar systematic reviews and meta-analyses were also searched and the PubMed ‘related articles’ feature was used for all identified trials and references of identified papers were checked for additional material cited ·       They included 10 studies comparing pH in ABG and VBG comparing 1,252 data points, six studies comparing pCO2, seven studies comparing bicarbonate, and one study comparing base deficit ·       Studies only reporting correlation or regression equations were not included as agreement is the clinically relevant end-point	·       Examines whether VBG analysis agrees with ABG in adult patients in the emergency room setting. ·       The researcher conducted separate literature reviews for the components of ABG and VBG analyses (pH, pCO2, bicarbonate) and base excess	·       The pooled mean difference between arterial and venous pH was 0.035 ·       The pooled mean difference between arterial and venous CO2 levels was 5.7 ·       The pooled mean difference between arterial and venous bicarbonate levels was -1.41 ·       They found that there is sufficient agreement between ABG and VBG when it comes to pH, bicarbonate, and base deficit in non-shock states. ·       For pCO2, however, the 95% limits of agreement were outside of the clinically acceptable range for VBG to serve as a substitute for ABG. ·       There seemed to be some merit in the use of VBG in screening for hypercarbia with respectable diagnostic accuracy, but this must be further evaluated	·       One limitation of this study is that it could not generalize the results for shock states ·       Additionally, it did not investigate pO2, though that is to be expected due to the low oxygenation of venous blood in comparison with arterial ·       Another problem with this study is that it was published in Australia, which is not ideal because it poses some doubt for the generalizability of the results
Byrne, A. L., Bennet, M., Chatterji, R., Symons, R., Pace, N. L, & Thomas, P. S.   (2014)	Systematic review and meta-analysis	·       A search was conducted of electronic databases as well as hand-searching of journals and reference lists through December 2012 to identify studies comparing PVBG with ABG analysis in adult subjects ·       Electronic databases from the Cochrane library, Cochrane register of diagnostic test accuracy studies, PubMed, KoreaMed, Google Scholar and TRIP databases were searched using a combination of following terms: venous blood gas, arterial blood gas, PVBG, ABG, VBG. The Boolean operators; AND as well as OR were used where appropriate. ·       In addition, a manual search of the following journals published between January 1995 and December 2012 was performed; Respirology, Annals of Emergency Medicine, European Journal of Emergency Medicine, Journal of Emergency Medicine, Emergency Medical Journal and Thorax ·       The researchers included 16 studies with 1768 participants in assessing pCO2, 11 studies with 1151 participants in assessing pO2, and 15 studies with 1747 participants in assessing pH.	·       Determine whether peripheral VBG has similar enough results as ABG in terms of pH, pCO2, and pO2	·       The estimated mean difference between the venous and arterial pH was 0.033 (CI 0.029– 0.038) ·       The estimated mean difference between venous and arterial PCO2 was  -3.88 (CI -5.35, -2.42) ·       The estimated mean difference between the venous and arterial PO2 was 36.78 (CI 25.63, 47.93) ·       They found that pH in VBG samples compared well with ABG samples. ·       VBG pO2 measurements compared poorly with ABG samples, however. This was to be expected, given the oxygen concentration in arterial blood versus venous blood. ·       While they did not analyze it, the researchers mentioned that pulse oximetry may be valuable in bridging this gap. ·       They also found that there was no statistically significant different between VBG and ABG pCO2 measurements, but the difference was substantial enough to be clinically significant. ·       They acknowledged though that VBG is helpful in determining if the patients are hypercapnic or hypoxic	·       One limitation of this analysis is the large heterogeneity of the samples; this makes the results a bit less reliable because it means their sample was widely different ·       Another is that the studies they included in their analysis could not be randomized, so there is the question of collection bias ·       This study was published in Australia, which is less than ideal concerning generalizability to the local population, but some of the researchers did have affiliation with institutions in the United States, which I thought might help the validity of the results in this regard.
Boulain, T., Garot, D., Vignon, P, Lascarrou, J. B., Benzekri-Lefevre, D., Dequin, P. F., Clinical Research in Inensive Care and Sepsis (CRICS) Group.   (2016)	Prospective cohort study	·       The researchers conducted this study in 10 medical–surgical adult ICUs ·       Consecutive patients with circulatory failure were included within 6 h after inclusion criteria were met ·       Patients were included if they had circulatory failure within 12 h of ICU admission and had an intra-arterial and superior vena cava (internal jugular or subclavian) catheter ·       Patients were not included in instances of brain death or if imminent death was expected ·       590 patients enrolled à 393 randomly assignment to derivation set à 197 randomly assigned to the validation set	·       This study assessed whether central venous blood gas analysis, clinical data, and peripheral oxygen saturation could be modelled to predict arterial values measured in intensive care unit patients with circulatory failure	·       In the validation set, the agreement intervals between predicted and actual values were −0.078/+0.084 units for arterial pH, −1.32/+1.36 kPa for arterial carbon dioxide partial pressure, −5.15/+4.47% for arterial oxygen saturation, and −1.07/+1.05 mmol litre−1 for arterial lactate (i.e. around two times our predefined clinically tolerable intervals for all variables). ·       This led to ∼5% (or less) of extreme-to-extreme misclassifications, thus giving our predictive models only marginal agreement. ·       Thresholds of predicted variables (as determined from the derivation set) showed high predictive values (consistently >94%), to exclude abnormal arterial values in the validation set ·       Although arterial pH, carbon dioxide partial pressure, and lactate could be predicted reasonably, arterial oxygen saturation was less reliable, and limits of agreement were all outside the authors’ predefined criteria for accuracy	·       SaO2 measurement method was not standardized in all 10 centers; this could have weakened the predictive function ·       The use of multiple linear regression s greatly diverged from the chemical and biological models regulating acid-base and oxygenation, so the predictive models might not have been as useful

Conclusion(s):

Article 1: Venous pH and pCO2 were reliable in predicting arterial values, but venous pO2 alone was not sufficient in predicting arterial pO2. There was a strong correlation between pulse oximetry reading and arterial pO2. Therefore, VBG in combination with pulse oximetry may serve as a reliable alternative to ABG.

Article 2: They found that there is sufficient agreement between ABG and VBG when it comes to pH, bicarbonate, and base deficit in non-shock states. For pCO2, however, the 95% limits of agreement were outside of the clinically acceptable range for VBG to serve as a substitute for ABG. There seemed to be some merit in the use of VBG in screening for hypercarbia with respectable diagnostic accuracy, but this must be further evaluated.

Article 3: The researchers found that pH in venous samples compared well to arterial samples. Additionally, there was no statistical difference in pCO2 values between venous and arterial samples, but there was a clinical significance. Venous measurements did not correlate with arterial pO2 measurements.

Article 4: Although arterial pH, carbon dioxide partial pressure, and lactate could be predicted reasonably using VBG, arterial oxygen saturation was less reliable, meaning that VBG could not reliably predict oxygenation and replace ABG in that regard.

Clinical Bottom Line:

Although the use of VBG is growing in popularity in order to analyze acid-base status in patients in many different settings because it is faster, easier, and less invasive to the patient, the results of VBG analysis are sufficiently mixed and thus do not correlate closely enough with those of ABG to become a reliable alternative in all patients, particularly patients that may be unstable. Measurements of pH align well between VBG and ABG, but, while not statistically significant in most cases, measurements of pCO2 and pO2 differ enough to be clinically significant. These measurements were, however, accurate enough to determine hypercapnia and hypoxia in the patients studied.

In the case that VBG must be used, as in poor arterial circulation or poor patient tolerance of ABG, VBG should be supplemented with pulse oximetry to better quantify venous blood oxygenation.

Weight of Evidence:

I weighed the Kelly and Byrne articles most heavily in coming to an ultimate conclusion due to their notably larger sample sizes than the other two articles, as well as the consideration that, as systematic reviews, they are higher levels of evidence. It could not be overlooked, however, that those articles were both published outside of the United States, which is not ideal when trying to generalize for a local population. I also heavily weighed the Zeserson article despite its being a lesser level of evidence and small sample size because of its consideration of the supplementation of pulse oximetry in drawing a conclusion about the application of VBG over ABG.

Magnitude of Any Effects:

pH can comparably be measured using VBG. Although many sources in the literature showed no statistical significance in pCO2 levels measured via VBG and ABG, this difference was great enough to have clinical significance and thus the two blood gas readings cannot be interchangeable. pO2 was both statistically and clinically significantly different in VBG measurements from ABG measurements, such that VBG alone could not be used to assess oxygenation status compared to ABG. 

Clinical Significance:

While blood pH can be measured with sufficient accuracy through VBG, ABG remains the most reliable method of measuring pCO2 and pO2. VBG, however, can be a useful tool in detecting hypercarbic and hypoxic states in patients who may not be able to tolerate an ABG. When VBG must be used, as in the cases of poor collateral circulation, or when repeat blood gasses are required, a pulse oximetry reading should be supplemented to more accurately assess oxygenation status.

Other Considerations:

More higher level evidence should be conducted in the United States to explore the clinical benefit of using VBG over ABG since VBG is growing in popularity considering the comfort of the patient, skill required, and other factors. Future studies should also examine the combination of VBG and pulse oximetry to more accurately measure oxygenation status.

Moreover, most studies focus on critically ill patients in ICUs, but it might be helpful to conduct a trial of stable, non-critical patients to see how useful VBG might be outside of hypercapnic or hypercarbic states in determining a patient’s baseline. In general, more research should be done focusing on larger samples.